Hi. My name is Jared, and I'm the antagonist. Although none of you know that yet.
This is where things start to get tricky. That funny little space between the beginning and the end commonly known as, The Middle.
Now, for the purposes of this discussion, (and to make this little series last the whole month.), we're going to focus on the front half of the middle. Let's say, from 25% to 50%. A normal story, works like you see below. You have the exposition, or the introduction, where you introduce the characters, setting, and occasionally the beginning of the conflict. In a novel, you have a lot more space to work with, so you'll generally go about, allowing the readers to get a feel for the characters, while still trying to drop a hint here or there to keep things interested and to provide a little bit of foreshadowing. In current novels, this introduction section should never be more than 25% of the book. Older novels might stretch that to 35 or even 50%, but that's asking a lot from today's audience who are used to 15 second commercials, 24 hour delivery, and 2 gigabyte download speeds.
After the introduction, is the rising action part of the story. In a novel, this section can easily stretch upwards of 25,000 words by itself. This can also be the hardest part of a novel to write. After all, you've introduced the characters, the setting, maybe the start of the conflict, but it's nowhere near time for the bad stuff to happen and you've got a long ways to go to get to that point. So what do you fill the time with so that you can hold your reader's attention and lead them along without getting bored?
Well, as the graph says, this is the start of the rising action. It's not like you can't do things here and there to keep things moving. In a horror movie, this is the point where you get the little things, like lights flicking on and off, maybe a door opens and closes when nobody's watching. A little further up the hill, the characters start to notice things themselves, but either nobody believes them, or they don't connect what happens to any impending sense of peril.
This is also the part where you fill in the world around your characters, and you delve into any needed back-story. Things which are important to understanding the characters, but which weren't necessarily Introduction material. After all, on a first date, you wouldn't try to impress someone with stories of your weird uncle who keeps over 100 named cockroaches as pets in his bedroom, or that your ex got a two-year prison sentence for assault just under two years ago.
So you've got relevant back-story, a few creaks and groans in the night, that isn't enough by itself to fill the space, so what else? Well, actually, that should do it. Remember, you're not just showing how the characters are reacting to what happens to them, you also need to establish the world around them and how the world reacts to the characters reactions. This goes a long way to making the story more believable, allowing it to better draw emotions out of the reader. For example, zombies are slowly making their way into a small town. The main character finds and kills one outside a local store. Obviously there are going to be witnesses, as well as no small amount of blood on the character's hands. The question is, if the rest of the town isn't aware of the zombies, how would they react to this otherwise bloody murder that just happened in front of them? Assume there was a good reason and go back to their business like nothing happened, or are they likely to call the cops? If they don't call the cops, either because they knew it was a zombie or some other reason, it better be explained and believable, and not something like; "Oh, the guy was a prick anyway, he had it coming."
The last bit I want to go over is escalation. Remember, this is rising action. Things need to be progressive. If you have a massive first scene followed by a long period of quiet, it better be explained and for a good reason. Having things escalate helps to create a sense that things are getting worse, as opposed to getting better. There's a reason you see movies like Paranormal Activity start with rattling pots and pans, move up to doors slamming, and then we see the characters getting flung through the air. If it went in the opposite order, it would be calming down to nothing, and there would be no final climax to worry about.
Sorry about the mix-up, I'll be back later, say, around 3 AM.
With all that, you shouldn't have any real trouble keeping things interesting for the second quarter of your book. And, if done right, you'll have characters that are fully fleshed out, believable, and that the readers care about by the mid-point of your story. At that point, as the author, you should be ready for all hell to break loose.